The present invention relates to a phase-locked loop (PLL) circuit, and more particularly, to a PLL circuit used, for example, to reproduce a write clock signal of an optical disc.
An optical disc is known as a recording medium that records data. For example, a digital versatile disc-recordable (DVD-R) is an optical disc that may be recorded by a user. The DVD-R typically includes a spiral pregroove (guide groove) and land prepits (hereafter referred to as LPPs) formed near the pregroove.
The pregroove wobbles along the optical disc. Wobble components include information related with absolute time (absolute position on the optical disc) and information related with the specification of the disc. Based on the absolute time information, data is recorded on the optical disc while recognizing the position of the recorded data on the optical disc.
The LPPs are formed at predetermined intervals along the pregroove. An LPP signal is generated in accordance with the reproduction of the LPPs. In accordance with the interval of the LPPs, the LPP signal has a pulse for every 16 pulses of a signal obtained from the wobble components (i.e., wobble signal).
To improve the accuracy for recording data to the optical disc, it is preferred that the timing for recording data to the optical disc be synchronized with the rotation of the optical disc. For example, data may be recorded in accordance with a clock signal obtained by reproducing a signal recorded on the optical disc. In this case, the area occupied by a single bit of recording data on the optical disc is always the same.
A phase-locked loop (PLL) circuit has been conventionally used. The PLL circuit has a phase comparator that compares a clock signal, which is generated by a voltage-controlled oscillator (VCO), with a pulse signal, which is to be synchronized with the clock signal. A voltage corresponding to the frequency difference and phase difference of the two signals is fed back to the VCO. The VCO then synchronizes the clock signal and the pulse signal.
Since the wobble signal facilitates synchronization, the wobble signal may be used by the PLL circuit to generate the clock signal in synchronism with the rotation of the optical disc.
To generate a clock signal that is further accurately synchronized with the rotation of the optical disc, it is preferred that the LPP signal be used instead of the wobble signal. However, the pulse cycle of the LPP signal is longer than that of the wobble signal. Thus, it is difficult for the PLL circuit to generate a clock signal that is accurately synchronized with the LPP signal.
The same problem occurs when the PLL circuit generates a clock signal that is to be synchronized with a signal having a relatively long pulse cycle (low frequency). It is difficult to synchronize such a long pulse cycle signal with the clock signal.